1. Field of the Invention
The present invention relates to boundary acoustic wave devices using boundary acoustic waves propagating along a boundary surface between different media and methods for manufacturing the boundary acoustic wave devices. The present invention particularly relates to a boundary acoustic wave device including a plurality of boundary acoustic wave elements integrated in one chip and a method for manufacturing the boundary acoustic wave device.
2. Description of the Related Art
Surface acoustic wave devices have been widely used for RF band-pass filters for mobile phones. The surface acoustic wave devices include piezoelectric substrates on which surface acoustic waves propagate. Surfaces of the piezoelectric substrates need to have cavities. Therefore, the surface acoustic wave devices have a complicated package structure and a large size.
On the other hand, boundary acoustic wave devices using boundary acoustic waves propagating along a boundary surface between different media have been attracting much attention. Since the boundary acoustic waves propagate along a boundary surface therebetween, the boundary acoustic wave devices need not be packaged so as to have cavities and therefore can be downsized.
Japanese Unexamined Patent Application Publication No. 2004-159262 discloses an exemplary boundary acoustic wave device. FIG. 9 is a front sectional view of the boundary acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2004-159262.
The boundary acoustic wave device 101 includes a first boundary acoustic wave element 102, a second boundary acoustic wave element 103, and a third boundary acoustic wave element 104 in series. The first boundary acoustic wave element 102 includes a first solid layer 102a made of a piezoelectric material; a second solid layer 102c, made of a piezoelectric or insulating material, overlying the first solid layer 102a; and IDT electrodes 102b sandwiched between the first and second solid layers 102a and 102c. The second boundary acoustic wave element 103, as well as the first boundary acoustic wave element 102, includes a first solid layer 103a, a second solid layer 103c, and IDT electrodes 103b sandwiched therebetween. The third boundary acoustic wave element 104, as well as the first boundary acoustic wave element 102, includes a first solid layer 104a, a second solid layer 104c, and IDT electrodes 104b sandwiched therebetween.
In the boundary acoustic wave device 101, the first, second, and third boundary acoustic wave elements 102, 103, and 104 are laminated and therefore can be mounted within a small space on a mounting board. This allows the boundary acoustic wave device and an electronic apparatus including the boundary acoustic wave device to be downsized.
The boundary acoustic wave device 101 has a configuration in which the first, second, and third boundary acoustic wave elements 102, 103, and 104 are arranged in the thickness direction thereof. Therefore, although a space for mounting the boundary acoustic wave device 101 is small, the boundary acoustic wave device 101 has a large thickness and therefore is unsuitable for thin apparatuses.